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Preferential battery supply for a load (LED etc.)

Main project requirements:

  1. Due to the availability of "free" energy in the form of solar-charged batteries, it is necessary to set up an automatic system where energy is drained first from the battery, and in case of discharge, automatically switched to AC-DC power adapter.

  2. In my case, the energy consumer is 12V LED light.

Schematic of the device.
 
Preferential battery supply, schema
Preferential battery supply
How it works and components used in the project.

  1. The circuit is turned on by turning on the AC-DC adapter, which turns on the Q1 mosfet.

  2. Zenner diode at 8.13V is used to reduce the voltage coming to the MOSFET gate in order to discharge the capacitors in the AC-DC adapter together with resistor R2(150R) and to turn off the MOSFET faster at power off (it is necessary for the situation when LEDs are powered from the battery).

  3. N-channel mosfet Q1 (IRF3205) is needed to create a closed circuit from the battery through the relay coil and the mosfet.

  4. If the battery is discharged or absent, the DPDT relay is not enabled and current flows through the normally closed contacts by default powering the LED by AC-DC adapter.

  5. If the battery is connected and charged, switching on the relay causes the relay contacts to switch and the load (LED) is powered from the battery.

  6. The DPDT relay coil operates from 9.5V (rated at 12V). I used a relay with two pairs of contacts. For this project, the second pair is not used.

  7. Diode 1n4007 is used to protect the MOSFET when power to the relay coil is interrupted.

  8. Schottky diode 1n5820 is used to avoid cyclic on/off of the relay.

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You can find a list of used components in this project with references in the Google-document ("Preferential battery supply" sheet).

 
 
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